Lithium ion secondary batteries, which are non-aqueous electrolyte secondary batteries, are used as the power source for small mobile devices such as mobile telephones, video cameras, personal digital assistants (PDA) and the like. Furthermore, research and development of large-scale lithium ion secondary batteries for use in automobiles such as hybrid cars is underway. From such a background, higher capacity and battery output characteristics for lithium ion secondary batteries are needed, and particularly, for lithium ion secondary batteries for used in an automobile, batteries having life characteristics that can be used over a longer period of time than consumer use batteries are further required.
Lithium nickel composite oxide (LNO), which is one cathode material for a lithium ion secondary battery, has advantages over mainstream lithium cobalt oxide (LCO) in that it has a high capacity, the raw ingredient nickel (Ni) is less expensive than cobalt (Co), and is stably available, so it is expected to be used as the next generation of cathode material, and thus research and development thereof is actively continuing.
However, lithium nickel composite oxide has problems in that the crystal stability is low, and there are problems with cycling characteristics and thermal stability.
In order to solve these problems, improvement of the battery characteristics using various additional elements is being investigated. For example, in JP 9-270258 (A), improvement of durability by adding Co to LiNiO2 is disclosed. However, even in a battery with the best durability, the capacity decreases by half after about 500 cycles, and in the case of use in an automobile, that durability is not sufficient.
On the other hand, as an improvement by concentration distribution of added elements, JP 2001-243948 (A) discloses a cathode active material for a lithium ion secondary battery with the object of improving the cycling characteristics by improving the structural stability that is characterized by using a LiCoO2 core and distributing a metal selected from among the group of Al, Mg, Sn, Ca, Ti and Mn at different concentration gradients from the surface to the center of the core. However, the object of this cathode active material for a lithium ion secondary battery is to improve the cycling characteristics of the battery, and does not take into consideration at all the improvement of safety of the battery. Moreover, it also dos not take into consideration application to a lithium nickel composite oxide, so the result in that case is unclear.
In order to improve the safety of lithium nickel composite oxide, JP 2008-166269 (A) proposes a lithium nickel composite oxide that is characterized by having Co and Al inside the particles, and having a concentration gradient of Mn concentration with respect to the radial direction of the particles, and with the Mn concentration being higher on the particle surface than in the center of the particle. However, in a battery that uses this lithium nickel composite oxide, resistance due to storage increases by 7% or more in one week, and when considering usage in an automobile, that durability is not sufficient.
There are particularly high expectations for the use of lithium ion secondary batteries as large-scale batteries for use as the power source for hybrid automobiles and electric automobiles, however, in this case, ensuring durability over a long period of time is essential. However, currently there is no lithium ion secondary battery that has both high safety and durability while at the same time having a high discharge capacity.